This invention generally relates to alkaline electrochemical cells. More particularly, this invention is concerned with alkaline cells having an anode that contains blended zinc powder.
Cylindrically shaped electrochemical cells are suitable for use by consumers in a wide variety of devices such as flashlights, radios and cameras. Batteries used in these devices typically employ a cylindrical metal container to house two electrodes, a separator, a quantity of electrolyte and a closure assembly that includes a current collector. Typical electrode materials include manganese dioxide as the cathode and zinc as the anode. An aqueous solution of potassium hydroxide is a common electrolyte. A separator, conventionally formed from one or more strips of paper, is positioned between the electrodes. The electrolyte is readily absorbed by the separator and anode.
Commercially available cylindrical alkaline batteries use an anode that includes zinc in particulate form. The anode is a gel that has absorbed an aqueous electrolyte. The zinc particles are uniformly dispersed within the gel so that particle-to-particle contact establishes an electrically conductive matrix throughout the anode. A current collector contacts the zinc and provides an electrically conductive path between the anode and one of the cell's terminals.
Due to the ever present desire to provide consumers with better performing batteries, battery engineers are constantly striving to improve the length of time that a battery will power a consumer's device. At the same time, the need to reduce the cost of the cell, while maintaining the cell's service performance, has become paramount. Previous attempts to improve service have included modifications to the zinc's alloy, changes to the shape of the zinc particles, modifications to the zinc powder's particle size distribution and forming free flowing zinc particles into agglomerates of zinc. For example, U.S. Pat. No. 6,022,639 describes incorporating zinc flakes into zinc powder that is used in an alkaline electrochemical cell. Unfortunately, the use of zinc flakes is known to substantially increase the viscosity of the gelled anode containing the zinc flake. The increase in viscosity can cause processing problems that adversely impact the efficiency of the cell manufacturing process. U.S. Pat. No. 6,284,410 describes adding zinc fines or zinc dust to a zinc powder. The addition of fines or dust is described as beneficial to the cell's overall performance. However, adding the fines or dust is known to increase the viscosity of the gelled anode which can cause processing problems in the cell manufacturing process. Numerous attempts have been made to improve the cell's run time by alloying one or more elements, such as bismuth, indium, aluminum, lead, tin, or manganese, with the zinc. Alloying elements with zinc has proven effective at increasing the service of the cell but the alloying process incurs additional expense at the zinc manufacturing facility which ultimately increases the cost of the cell. Forming zinc agglomerates from free flowing zinc powder is described in U.S. Pat. No. 7,332,247. While agglomerates do improve the cell's run time on certain service tests, the process steps needed to form the agglomerates also increases the cost of the zinc and thus the cost of the battery. All of the changes to the zinc described above can be collectively described as an industry wide effort to engineer the zinc powder to improve cell performance by altering certain physical and/or chemical characteristics of the zinc powder. However, there continues to be a need for further advances in the development of zinc powders that will enable service improvements while minimizing any additional cost to the cell or, alternatively, reducing the cost of the zinc while maintaining the cell's service.